John Flanagan 
Ranger’s Apprentice The Royal Ranger 4: The Missing Prince [EPUB ebook] 

King Philippe of Gallica appeals to King Duncan for help. His son is being held hostage by a power-hungry baron, and Philippe wants Duncan to send a Ranger to rescue the boy. Will and Maddie are perfect for the task. Travelling as father and daughter to allay suspicion, they will pose as entertainers to infiltrate the Chateau des Falaises, where the prince is being held. But travelling through Gallica has its own risks: armed bandits, wild weather and even a mysterious follower all have to be faced and overcome. Will they ever find the prince? And if they do, how will they rescue him from the evil Baron Lassigny?